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Sr Isotopic Variations Along the Juan De Fuca Ridge

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University of South Florida Scholar Commons Marine Science Faculty Publications College of Marine Science 9-1984 Sr Isotopic Variations along the Juan de Fuca Ridge Jacqueline Eaby U.S. Geological Survey, [email protected] David A. Clague U.S. Geological Survey John R. Delaney University of Washington Follow this and additional works at: https://scholarcommons.usf.edu/msc_facpub Part of the Life Sciences Commons Scholar Commons Citation Eaby, Jacqueline; Clague, David A.; and Delaney, John R., "Sr Isotopic Variations along the Juan de Fuca Ridge" (1984). Marine Science Faculty Publications. 1321. https://scholarcommons.usf.edu/msc_facpub/1321 This Article is brought to you for free and open access by the College of Marine Science at Scholar Commons. It has been accepted for inclusion in Marine Science Faculty Publications by an authorized administrator of Scholar Commons. For more information, please contact [email protected]. JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 89, NO. B9, PAGES 7883-7890, SEPTEMBER 10, 1984 Sr IsotopicVariations Along the Juande Fuca Ridge JACQUELINEEABY AND DAVID A. CLAGUE U.S. GeologicalSurvey, Menlo Park, California JOHN R. DELANEY OceanographyDepartment, University of Washington,Seattle Srisotopic ratios of 39glass and microcrystalline basalt samples along the Juan de Fuca Ridge and 1 glasssample from BrownBear Seamount are at the lowerend of the rangefor normalmid-oceanic ridge basalt(MORB)' the average87Sr/S6Sr ratio is 0.70249+ 0.00014(2-a). Althoughsubtle variations exist alongstrike of the ridge,the Sr isotopedata do not showsystematic variation relative to the proposed CobbHotspot. The isotopicdata are inconsistentwith an enrichedmantle-plume origin for the Cobb- EickelbergSeamount chain, as has beenproposed for Iceland,the Azores,and the Galapagosspreading center.Sr isotopicratios of samplescollected north and southof the Cobb offsetare identical,although minor elementratios indicate that theseregions have chemicallydistinct mantle sources. These distinct mantlesources may not havebeen separated long enough to developisotopic differences. INTRODUCTION characterizedby high 2ø6pb/2ø'*Pb,low 87Sr/86Sr,and high The hotspo•tmodel proposed by Wilson[1965] and Morgan •'•3Nd/•'•'•Nd.Geochemical variations of lavasalong hotspot/ [!973] explainsthe origin of linear seamountchains as the ridge systemsare interpretedto be the result of mixing be- result of movementof lithosphericplates over mantle plumes. tween enriched and normal MORB end-members [Schillin•t, The Cobb-EickelbergSeamount chain, in the northeastPaci- 1975a; Schillin•tet al., 1982a]. fic, trends northwestward from the Juan de Fuca Ridge and is The objectiveof thisstudy is to presentnew Sr isotopicdata interpretedto have a hotspot origin. A seamountat approxi- for lavas dredgedfrom the Juan de Fuca Ridge and to use these data to evaluate the mantle source characteristics of the mately lat 46øN on the axis of the Juan de Fuca Ridge (axial seamount)is coincidentwith the intersectionof the Cobb- Cobb Hotspot. EickelbergSeamount chain and the ridge; it is thought to be GEOLOGIC SETTING the site of presenthotspot activity, herein referred to as the CobbHotspot [Vogt and Johnson, 1975]. The Juande Fuca Ridge,which extends 500 km from the Detailed studiesof other mid-oceanicridge/hotspot systems, BlancoFracture Zone to the SovancoFracture Zone (Figure suchas Iceland [Hart et al., 1973; $chillin•h1973, 1975a], the 1),is a spreadingcenter of moderate rate, 30 •ma-x between Azores [White et el., 1975, 1976; $chillin•h 1975b], and the the Pacificand Juan de Fucaplates. Mor•tan [1973] proposed Galapagosspreading center [$chillin•l et el., 1982a;Verma and theexistence of a •otspoton theJuan de FucaRidge to Schilling,1982] showed that basalt erupted along deep mid- explainthe northwest trending linear seamount chain includ- oceanicridge segments(normal MORB) differs from basalt ingBrown Bear, Cobb, and Eickelberg seamounts. K-Ar ages erupted along topographic highs associatedwith islands fromHorton Seamount, located near the northwest end of the astride the ridge (enriched MORB). Normal MORB is seamountchain, indicate that the Cobb Hotspot has been a characterizedby low concentrationsof incompatibleelements; zoneof high volcanicproductivity for at least20 Ma [Turner light rare earth elementdepleted patterns; high K/Ba, K/Rb, et el., 1980]. and Zr/Nb ratios; low 8•Sr/S6Sr ratios; and high There are four morphologicallydistinct segmentsof the •'•3Nd/X'•'•Ndratios [White et el., 1975;DePaolo and Wass. Juande Fuca Ridge(Figure 2)'(1) A 90-km-long,linear, sym- erburg,1976]. These chemical characteristics are consistentmetrical southern ridge segmentnorth of the BlancoFracture with those of basalt derived from mantle sourceregions deple- Zone;(2) a 140-km-longcomplexly disturbed central segment, ted by previousmelting events[Kay et el., 1970]. Enriched recentlyoffset approximately 20 km westof the magneticaxis MORB, which has chemical affinities to oceanic island tho- of symmetry,which includes the axial seamount;(3) another leiites, differs from normal MORB by having higher con- 90-km-long,linear, symmetricalnorthern ridge segmentex- centrationsof incompatible elements;rare earth element pat- tendingnorthward to the Cobboffset (the tip of a northward ternsmore enriched in thelight elements; lo•wer K/Ba, K/Rb, propagatingrift [Johnsonetal., 1983]); and (4) the ridge north and Zr/Nb ratios; higher 8*Sr/8aSr ratios; and lower ofthe Cobb offset, referred to asthe Endeavor Ridge [Delaney x'•3Nd/X'•'•Ndratios than normalMORB [Bryanet el., 1976; et al., 1982]. White and Bryan, 1977; O'Nions et el., 1977]. Schilling[1973, Basalt collectedsouth of the Cobb offset (regions 1-3) is 1975b] and Schillinget el. [1982a] proposethat enriched predominan,tlyaphyric olivine- to quartz-normativetholeiite MORB is generatedfrom a nondepletedmantle source.Zind- slightlyto distinctlyenriched in iron and titaniumcompared let et el. [1982] propose a third chemically independent with normalMORB. Regionalvariations of FeO* (total iron mantle component,possibly related to recycledoceanic crust, calculatedas FeO) are plotted as a functionof latitude in Figure3. Ferrobasalt {> 10% FeO*) is particularly abundant Thispaper is not subjectto U.S.copyright. Published in 1984by at the southernend of region 1, and 20-30 km southof the the AmericanGeophysical Union. Cobb offset. The maximum iron and titanium enrichment in Paper nurlaber4B0708. the lavasoccurs behind zones identified as propagatingrifts in 7883 7884 EABY ET AL.' SR ISOTOPIC RATIOS ALONG JUAN DE FUCA RIDGE chemistry,and glasschemistry of a muchlarger set of samples will be discussedelsewhere (J. R. Delaney et al., unpublished data, 1984).All samplesare glassybasalt from the actively spreadingaxis and werechosen for their fresh•:essand spatial distribution.Samples along the ridgeand from the peak of the • I -• WelkerGuyol axial seamountare approximately25 km apart. ß • ß , •t • ANALYTICAL TECHNIQUES .- . •. ,' ', - • Hand-picked,fresh glasschips were rinsedin acetoneand __ ' • • ' ' ' •m,Chain • 'O DIlwood . washed with distilled water in an ultrasonic cleaner for 5 min. Microcrystalline-basaltsamples were ground to < 200 meshin .... , -- .'• i • a tungstencarbide shatterbox.Samples of powder or glass I. '/ ....,, , •o•LL/• :5 • 'a weighingapproximately 100 mg were dissolvedin HF and HC10,•, and Sr was separatedfrom the unspikedaliquots i i '1 '1 usingstandard ion-exchange chromatography. Mass analysis was done at the U.S. Geological Surveyin Menlo Park on a / . ' ' "•,• •'Y • • I • • 40øN / I '' I 1 •1 •1 I •.,..:S'o,."?.•I I 49 ø N 150ø 1400 I •0 ø 1200W Fig. 1. Seamount and ridge distribution in the northeast Pacific Ocean. The youngest seamount of the Cobb-Eickelberg Seamount chain occurs at the intersection of the seamount chain and the Juan de Fuca Ridge. Figure modified from Barr [ 1974]. %0 ENDEAVOUR . both regions [Hey and Wilson, 1982; Delaney et al., 1981; SEGMENT - 48øN Johnsonet al., 1983]. The petrologic diversity of lavas in re- (region 4) gions 1-3 may be the result of variable degreesof fractional crystallization[Clague and Bunch,1976] in magma chambers spatiallyassociated with propagatingrifts [Sintonet al., 1983]. COBB OFFSE'T Vogt and Byefly [1976] postulated that the iron and titanium enrichmentcould result from fractional crystallizationin a 47øN NORTHERN subaxial conduit transporting magma longitudinally away SEGMENT from the hotspot. (region 3) Basalt from region 4 is chemicallydistinct from basalt south of the Cobb offset. The lavas are not unusuallyenriched in iron and titanium but are enrichedin K20 (up to 0.56 wt %), Na20 (up to 3.36 wt %), and SiO: (up to 52.5 wt %). The CENTRAL P:Os/K:O ratios rangefrom 0.5 to 0.8 comparedwith P:O5 SEGMENT 46øN /K20 ratios of > 1.0 for lavas south of the Cobb offset (region 2) (J.R. Delaney et al., unpublisheddata, 1984).The P:Os/K:O ratios are probably indicative of mantle source regions [Clagueet al., 1981-1,thus suggestingthat lava from region4 is 4R derived from mantle sources distinct from those of lava from regions 1-3. 46 45 øN SAMPLE SELECTION A comprehensivesampling and surveyingprogram of the 4 500-km-long ridge segmentbetween the Sovancoand Blanco I 'if3 i n•q t2(1 124 fracturezones was initiated by the Universityof Washington in 1980 to characterizethe chemical,petrologic, and tectonic interaction among a spreading center of moderate rate, a I 44øN 131 o ',1/ 128oW 127ø w seamount-generatinghotspot, and propagatingrifts. Ratios of 87Sr/86Srwere determined
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  • A 65 K.Y. Time Series from Sediment-Hosted Glasses Reveals Rapid Transitions in Ocean Ridge Magmas

    A 65 K.Y. Time Series from Sediment-Hosted Glasses Reveals Rapid Transitions in Ocean Ridge Magmas

    A 65 k.y. time series from sediment-hosted glasses reveals rapid transitions in ocean ridge magmas David J. Ferguson1,2*, Yinqi Li1,3, Charles H. Langmuir1, Kassandra M. Costa4,5, Jerry F. McManus4,5, Peter Huybers1, and Suzanne M. Carbotte4 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA 2School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK 3School of Earth Sciences, Zhejiang University, Hangzhou 310027, China 4Lamont-Doherty Earth Observatory (LDEO), Columbia University, Palisades, New York 10964, USA 5Department of Earth and Environmental Sciences, Columbia University, New York, New York 10027, USA ABSTRACT 2013; Portner et al., 2015). The extent to which Studies of ocean ridge magmatism have been hampered by the difficulty in construct- such glasses might be preserved in older sedi- ing time-series data over more than a few thousand years. Sediment rapidly covers newly ments and over longer time periods, however, formed ocean crust, and older rocks, even when recovered from fault scarps, cannot be dated has not previously been demonstrated. Here we accurately. Ridge eruptions, however, disperse pyroclastic glass over distances as far as 5 show using a piston core that reached 600 ka km, and these glasses have been shown to persist for thousands of years in on-ridge sediment basement that ancient glasses are indeed pre- push cores. Here we present data on such glasses from a piston core that impacted basement served and permit high-resolution observations in much older (600 ka) sediment. The age of deposition was determined using established over tens of thousands of years.